Semiconductor device with source/drain contact formed using bottom-up deposition

ABSTRACT

A semiconductor device includes first and second semiconductor fins extending from a substrate, and first and second epitaxial layers wrapping around the first and second semiconductor fins, respectively. The semiconductor device further includes a contact plug over the first epitaxial layer and the second epitaxial layer. The contact plug includes a first interfacial layer over the first epitaxial layer and a second interfacial layer over the second epitaxial layer. The first and second interfacial layers include a noble metal element and a Group IV element.

PRIORITY CLAIM AND CROSS-REFERENCE

The present application is a Continuation Application of the U.S.application Ser. No. 17/107,471, filed Nov. 30, 2020, which is aDivisional Application of the U.S. application Ser. No. 16/252,405,filed Jan. 18, 2019, now U.S. Pat. No. 10,854,716, issued on Dec. 1,2020, which claims priority to U.S. Provisional Application Ser. No.62/711,640, filed Jul. 30, 2018, all of which are herein incorporated byreference in their entireties.

BACKGROUND

As the semiconductor industry has progressed into nanometer technologyprocess nodes in pursuit of higher device density, higher performance,and lower costs, challenges from both fabrication and design issues haveresulted in the development of three-dimensional designs, such as afin-type field effect transistor (FinFET) and the use of a metal gatestructure with a high-k (dielectric constant) material. The metal gatestructure is often manufactured by using gate replacement technologies,and sources and drains are formed by using an epitaxial growth method.Source/drain contact plugs are also formed on the sources/drain regions.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is flow chart illustrating a method of forming source/draincontacts in accordance with some embodiments.

FIGS. 2-16 show exemplary cross sectional views of various stages formanufacturing a FinFET according to some embodiments of the presentdisclosure.

FIGS. 17A, 18A, 19A and 20A are exemplary cross-sectional viewscorresponding to the line A-A illustrated in FIG. 16 at various stagesof forming contact plugs according to some embodiments of the presentdisclosure.

FIGS. 17B, 18B, 19B and 20B are exemplary cross-sectional viewscorresponding to the line B-B illustrated in FIG. 16 at various stagesof forming contact plugs according to some embodiments of the presentdisclosure.

FIGS. 21A, 22A and 23A are exemplary cross-sectional views correspondingto the line A-A illustrated in FIG. 16 at various stages of formingcontact plugs according to some embodiments of the present disclosure.

FIGS. 21B, 22B and 23B are exemplary cross-sectional views correspondingto the line B-B illustrated in FIG. 16 at various stages of formingcontact plugs according to some embodiments of the present disclosure.

FIG. 24A is an exemplary cross-section view corresponding to the lineA-A in FIG. 16 of a semiconductor device according to some embodimentsof the present disclosure.

FIG. 24B is an exemplary cross-section view corresponding to the lineB-B in FIG. 16 of a semiconductor device according to some embodimentsof the present disclosure.

FIGS. 25 and 26 are exemplary cross-sectional views corresponding to theline A-A illustrated in FIG. 16 at various stages of forming contactplugs according to some embodiments of the present disclosure.

FIG. 27 illustrates experimental results of depositing ruthenium onvarious materials using a CVD process according to some embodiments ofthe present disclosure.

FIG. 28 illustrates experimental results of depositing ruthenium onvarious materials using a CVD process according to some embodiments ofthe present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the some embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. As used herein,“around,” “about,” “approximately,” or “substantially” shall generallymean within 20 percent, or within 10 percent, or within 5 percent of agiven value or range. Numerical quantities given herein are approximate,meaning that the term “around,” “about,” “approximately,” or“substantially” can be inferred if not expressly stated.

Fins of fin-type field effect transistors (FinFETs) may be patterned byany suitable method. For example, the fins may be patterned using one ormore photolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers may then be used to pattern the fins.

FIG. 1 illustrates is a method of forming semiconductor devices inaccordance with some embodiments. The method may be implemented onplanar devices or multi-gate devices, such as FinFETs, gate-all-around(GAA) devices, Omega-gate (Ω-gate) devices, or Pi-gate (Π-gate) devices,as well as strained-semiconductor devices, silicon-on-insulator (SOI)devices, partially-depleted SOI devices, fully-depleted SOI devices, orother devices. FIGS. 2-16 show exemplary cross sectional views ofvarious stages for manufacturing a FinFET according to some embodimentsof the present disclosure. In the subsequent discussion, the processsteps shown in FIGS. 2-16 are discussed referring to the process stepsin FIG. 1 . It is understood that additional operations can be providedbefore, during, and after processes shown by FIGS. 2-16 , and some ofthe operations described below can be replaced or eliminated, foradditional embodiments of the method. The order of theoperations/processes may be interchangeable.

In the manufacturing method for a FinFET, fin structures are formed. Amask layer 10 is formed over a substrate 50, as shown in FIG. 2 . Themask layer 10 is formed by, for example, a thermal oxidation processand/or a chemical vapor deposition (CVD) process. The substrate 50 is,for example, a p-type silicon substrate with an impurity concentrationin a range from about 1×10¹⁵ cm⁻³ to about 1×10¹⁶ cm⁻³. In otherembodiments, the substrate is an n-type silicon or germanium substratewith an impurity concentration in a range from about 1×10¹⁵ cm⁻³ toabout 1×10¹⁶ cm⁻³.

Alternatively, the substrate 50 may comprise another elementarysemiconductor, such as germanium; a compound semiconductor includingGroup IV-IV compound semiconductors such as SiC and SiGe, Group III-Vcompound semiconductors such as GaAs, GaP, GaN, InP, InAs, InSb, GaAsP,AlGaN, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinationsthereof. The substrate 50 may include various regions that have beensuitably doped with impurities (e.g., p-type or n-type conductivity).

The mask layer 10 includes, for example, a pad oxide (e.g., siliconoxide) layer 11 and a silicon nitride mask layer 12, as shown in FIG. 2, in some embodiments. The pad oxide layer 11 may be formed by usingthermal oxidation or a CVD process. The silicon nitride mask layer 12may be formed by a physical vapor deposition (PVD), such as a sputteringmethod, a CVD, plasma-enhanced chemical vapor deposition (PECVD), anatmospheric pressure chemical vapor deposition (APCVD), a low-pressureCVD (LPCVD), a high density plasma CVD (HDPCVD), an atomic layerdeposition (ALD), and/or other processes. The thickness of the pad oxidelayer 11 is in a range from about 2 nm to about 15 nm and the thicknessof the silicon nitride mask layer 12 is in a range from about 2 nm toabout 50 nm in some embodiments.

A mask pattern is further formed over the mask layer. The mask patternis, for example, a resist pattern formed by lithography operations. Byusing the mask pattern as an etching mask, a hard mask pattern of thepad oxide layer 11 and the silicon nitride mask layer 12 is formed, asshown in FIG. 3 .

Then, as shown in FIG. 4 , by using the hard mask pattern as an etchingmask, the substrate 50 is patterned into fin structures 52 for an n-typeFET and fin structures 54 for a p-type FET by trench etching using a dryetching method and/or a wet etching method. The dimensions for the finstructures 52 for an n-type FET may be the same as or different fromthose for the fin structures 54 for a p-type FET.

In FIG. 4 , two fin structures 52 and two fin structures 54 are disposedover the substrate 50. However, the number of the fin structures is notlimited to two. The numbers may be as small as one or more than three.In addition, one or more dummy fin structures may be disposed adjacentboth sides of the fin structures 52 and/or the fin structures 54 toimprove pattern fidelity in patterning processes.

The fin structures 52, 54 may be made of the same material as thesubstrate 50 and may continuously extend or protrude from the substrate50. In this embodiment, the fin structures are made of Si. The siliconlayers of the fin structures 52, 54 may be intrinsic, or appropriatelydoped with an n-type impurity or a p-type impurity.

The width W1 of the fin structures 52, 54 is in a range from about 5 nmto about 40 nm in some embodiments, and is in a range from about 7 nm toabout 12 nm in other embodiments. The space S1 between two finstructures is in a range from about 10 nm to about 40 nm in someembodiments. The height H1 (along the Z direction) of the fin structures52, 54 is in a range from about 100 nm to about 300 nm in someembodiments, and is in a range from about 50 nm to 100 nm in otherembodiments.

The lower part of the fin structures 52, 54 may be referred to as a wellregion, and the upper part of the fin structures 52, 54, which iscovered by a gate electrode may be referred to as a channel or a channelregion, and the upper part of the fin structures 52, 54, which is notcovered by the gate electrode may be referred to as a source and adrain, or a source region and a drain region. In this disclosure,“source” and “drain” may be collectively referred to as “source/drain.”The height of the well region is in a range from about 60 nm to 100 nmin some embodiments, and the height of the channel region is in a rangefrom about 40 nm to 120 nm, and is in a range from about 38 nm to about60 nm in other embodiments.

After the fin structures 52, 54 are formed, a first protective layer 15is formed to cover the structures 52, 54, as shown in FIG. 5 . The firstprotective layer 15 is made of, for example, silicon oxide, siliconnitride (SiN) or silicon oxynitride (SiON). In an embodiment, the firstprotective layer 15 is made of SiN. The first protective layer 15 can beformed by CVD. The thickness of the first protective layer 15 is in arange from about 1 nm to about 20 nm in some embodiments.

After the first protective layer 15 is formed, a second protective layer17 is formed as shown in FIG. 6 . The second protective layer 17 is madeof, for example, silicon oxide, silicon nitride (SiN) or siliconoxynitride (SiON) and is different from the first protective layer 15.In some embodiments, the second protective layer 17 is made of siliconoxide. The second protective layer 17 can be formed by CVD. Thethickness of the second protective layer 17 is in a range from about 1nm to about 20 nm in some embodiments.

Further, an isolation insulating layer 58 is formed in spaces betweenthe fin structures and/or a space between one fin structure and anotherelement formed over the substrate 50, as shown in FIG. 7 . The isolationinsulating layer 58 may also be called a “shallow-trench-isolation(STI)” layer. The insulating material for the isolation insulating layer58 may include one or more layers of silicon oxide, silicon nitride,silicon oxynitride (SiON), SiOCN, fluorine-doped silicate glass (FSG),or a low-k dielectric material. The isolation insulating layer is formedby LPCVD (low pressure chemical vapor deposition), plasma-CVD orflowable CVD. In the flowable CVD, flowable dielectric materials insteadof silicon oxide may be deposited. Flowable dielectric materials, astheir name suggest, can “flow” during deposition to fill gaps or spaceswith a high aspect ratio. Various chemistries are optionally added tosilicon-containing precursors to allow the deposited film to flow. Insome embodiments, nitrogen hydride bonds are added. Examples of flowabledielectric precursors, particularly flowable silicon oxide precursors,include a silicate, a siloxane, a methyl silsesquioxane (MSQ), ahydrogen silsesquioxane (HSQ), an MSQ/HSQ, a perhydrosilazane (TCPS), aperhydro-polysilazane (PSZ), a tetraethyl orthosilicate (TEOS), or asilyl-amine, such as trisilylamine (TSA). These flowable silicon oxidematerials are formed in a multiple-operation process. After the flowablefilm is deposited, it is cured and then annealed to remove un-desiredelement(s) to form silicon oxide. When the un-desired element(s) isremoved, the flowable film densifies and shrinks. In some embodiments,multiple anneal processes are conducted. The flowable film is cured andannealed more than once. The flowable film may be doped with boronand/or phosphorous.

The insulating layer 58 is first formed in a thick layer as shown inFIG. 7 so that the fin structures 52 and 54 are embedded in the thicklayer, and the thick layer is recessed so as to expose the upperportions of the fin structures 52 and 54, as shown in FIG. 8 . Therespective step is shown as step S11 in the process flow in FIG. 1 . Theinsulating layer 58 can be recessed by using dry and/or wet etching. Insome embodiments, the mask layers 11 and 12 and the first and secondprotective layers 15 and 17 are also removed from the exposed portionsof the structures 52, 54.

The height H2 of the fin structures from the upper surface of theisolation insulating layer 58 is in a range from about 20 nm to about100 nm in some embodiments, and is in a range from about 30 nm to about50 nm in other embodiments. After or before recessing the isolationinsulating layer 58, a thermal process, for example, an anneal process,may be performed to improve the quality of the isolation insulatinglayer 58. In certain embodiments, the thermal process is performed byusing rapid thermal annealing (RTA) at a temperature in a range fromabout 900° C. to about 1050° C. for about 1.5 seconds to about 10seconds in an inert gas ambient, such as an N₂, Ar or He ambient.

In some embodiments, a gate replacement technology is employed. Afterthe insulating layer 58 is formed (and recessed), one or more dummy gatestructures each including a dummy gate dielectric layer 20 and a dummygate electrode layer 22 is formed over the fin structures 52, 54, asshown in FIG. 9 . The respective step is shown as step S12 in theprocess flow in FIG. 1 . As shown in FIG. 9 , the gate structure extendsin the X direction, while the fin structures extend in the Y direction.

To fabricate the dummy gate structure, a dielectric layer and a polysilicon layer are formed over the isolation insulating layer 58 and theexposed fin structures 52, 54, and then patterning operations areperformed so as to obtain the dummy gate structure including a dummygate electrode layer 22 made of poly silicon and a dummy gate dielectriclayer 20. In some embodiments, the polysilicon layer is patterned byusing a hard mask and the hard mask remains on the dummy gate electrodelayer 22 as a cap insulating layer. The hard mask (cap insulating layer)includes one or more layers of insulating material. The cap insulatinglayer includes a silicon nitride layer formed over a silicon oxide layerin some embodiments. In other embodiments, the cap insulating layerincludes a silicon oxide layer formed over a silicon nitride layer. Theinsulating material for the cap insulating layer may be formed by CVD,PVD, ALD, e-beam evaporation, or other suitable process. In someembodiments, the dummy gate dielectric layer 20 may include one or morelayers of silicon oxide, silicon nitride, silicon oxy-nitride, or high-kdielectrics. In some embodiments, a thickness of the dummy gatedielectric layer 20 is in a range from about 2 nm to about 20 nm, and ina range from about 2 nm to about 10 nm in other embodiments. The heightof the dummy gate structures is in a range from about 50 nm to about 400nm in some embodiments, and is in a range from about 100 nm to 200 nm inother embodiments.

If a gate-first technology is employed, the dummy gate electrode layer22 and the dummy gate dielectric layer 20 are used as a gate electrodeand a gate dielectric layer.

Afterwards, sidewall spacers 24 are formed on opposite sidewalls of thedummy gate electrode layer 22, as shown in FIG. 10 . Then, as shown inFIG. 11 , the sidewall spacers 24 formed on the fin structures 52, 54are removed by using, for example, anisotropic dry etching, so as toexpose source/drain regions of the structures 52, 54. The respectivestep is shown as step S13 in the process flow in FIG. 1 . The sidewallspacers 24 include one or more layers of insulating material, such asSiO₂, SiN, SiON, SiOCN or SiCN, which are formed by CVD, PVD, ALD,e-beam evaporation, or other suitable process. A low-k dielectricmaterial may be used as the sidewall spacers. The sidewall spacers 24are formed by forming a blanket layer of insulating material andperforming anisotropic etching. In an embodiment, the sidewall spacerlayers are made of silicon nitride based material, such as SiN, SiON,SiOCN or SiCN. The thickness of the sidewall spacers 24 is in a rangefrom about 2 nm to about 10 nm in some embodiments.

After the source/drain regions of the fin structures 52, 54 are exposed,source/drain epitaxial layers are formed, as shown in FIG. 12 . Therespective step is shown as step S14 in the process flow in FIG. 1 . Thesource/drain epitaxial layer 60 for the n-type FET includes one or morelayers of semiconductor material, such as, SiC, SiP and SiCP, whichprovides appropriate stress to the channel of the n-type FET, in someembodiments. When SiP or SiCP is used as multi epitaxial layers, thelayers have different P and/or C concentrations. Since the fin structure52 is a crystalline Si, the epitaxial layer 60 is also crystalline. Thesource/drain epitaxial layer 62 for the p-type FET includes one or moreof a semiconductor material, such as, Ge and Si_(x)Ge_(1-x) where 0<x<1,which provides appropriate stress to the channel of the p-type FET, insome embodiments. When SiGe is used as the multi epitaxial layers, thelayers have different Ge concentrations. Since the fin structures 52, 54are crystalline Si, the epitaxial layers 60, 62 are also crystalline. Incertain embodiments, a Group III-V material which provides appropriatestress to the channel is used as the source/drain epitaxial layers 60and/or 62. In some embodiments, the source/drain epitaxial layers 60and/or 62 include multiple layers of epitaxially formed semiconductormaterials.

The source/drain epitaxial layers 60, 62 may be grown at a temperatureof about 400 to 800° C. under a pressure of about 80 to 150 Torr, byusing a Si containing gas such as SiH₄, Si₂H₆ or SiCl₂H₂; a Gecontaining gas, such as GeH₄, Ge₂H₆ or GeCl₂H₂; a C containing gas, suchas CH₄ or C₂H₆; and/or a dopant gas, such as PH₃. The source/drainstructure for an n-type FET and the source/drain structure for a p-typeFET may be formed by separate epitaxial processes.

In the present disclosure, after the source/drain epitaxial layers 60,62 are formed, the source/drain epitaxial layer 60 formed on one finstructure 52 is not in contact with (i.e., physically separated from)the source/drain epitaxial layer 60 formed on the adjacent fin structure52 as shown in FIG. 12 . Similarly, the source/drain epitaxial layer 62formed on one fin structure 54 is not in contact with (i.e., physicallyseparated from) the source/drain epitaxial layer 62 formed on theadjacent fin structure 54 as shown in FIG. 12 . The space S2 between thesource/drain epitaxial layers 60 (or 62) is in a range from about 5 nmto 15 nm in some embodiments. The thicknesses of the source/drainepitaxial layers 60 and 62 are adjusted to secure the desired space S2depending on the space S1 (see, FIG. 4 ) between two fin structures.

After the source/drain epitaxial layers 60 and 62 are formed, adielectric cover layer 65 is formed over the source/drain epitaxiallayers 60 and 62 and a first interlayer dielectric (ILD) layer 67 isformed over the dielectric cover layer 65, as shown in FIG. 13 . Therespective step is shown as step S15 in the process flow in FIG. 1 . Thedielectric cover layer 65 is made of, for example, SiN or SiON, and hasa thickness in a range from about 2 nm to about 20 nm in someembodiments. The first ILD 67 is made of a different material than thedielectric cover layer 65 and is made of, for example, one or morelayers of silicon oxide, SiCN, SiOCN or a low-k material.

After the ILD layer 67 is formed, one or more metal gate structures areformed. The dummy gate structures (the dummy gate electrode layer 22 andthe dummy gate dielectric layer 20) are removed and replaced with metalgate structures. In certain embodiments, the first ILD layer 67 isformed over the dummy gate structures and a planarization operation,such as a chemical mechanical polishing (CMP) process or an etch-backprocess, is performed to expose the upper surface of the dummy gateelectrode layer 22. Then, the dummy gate electrode layer 22 and thedummy gate dielectric layer 20 are removed, by appropriate etchingprocesses, respectively, to form a gate opening. A metal gate structureincluding a gate dielectric layer 72, one or more work functionadjustment layers 73 and a metal gate electrode layer 74 are formed inthe gate openings, as shown in FIG. 14 . The respective step is shown asstep S16 in the process flow in FIG. 1 .

The gate dielectric layer 72 may be formed over an interface layer (notshown) disposed over the channel layer of the fin structures 52, 54. Theinterface layer may include silicon oxide or germanium oxide with athickness of 0.2 nm to 1.5 nm in some embodiments. In other embodiments,the thickness of the interface layer is in a range about 0.5 nm to about1.0 nm.

The gate dielectric layer 72 includes one or more layers of dielectricmaterials, such as silicon oxide, silicon nitride, or high-k dielectricmaterial, other suitable dielectric material, and/or combinationsthereof. Examples of high-k dielectric material include HfO₂, HfSiO,HfSiON, HfTaO, HfTiO, HfZrO, zirconium oxide, aluminum oxide, titaniumoxide, hafnium dioxide-alumina (HfO₂—Al₂O₃) alloy, other suitable high-kdielectric materials, and/or combinations thereof. The gate dielectriclayer 72 is formed by, for example, chemical vapor deposition (CVD),physical vapor deposition (PVD), atomic layer deposition (ALD), highdensity plasma CVD (HDPCVD), or other suitable methods, and/orcombinations thereof. The thickness of the gate dielectric layer is in arange from about 1 nm to about 10 nm in some embodiments, and may be ina range from about 2 nm to about 7 nm in other embodiments.

The one or more work function adjustment layers 73 are formed over thegate dielectric layer 72, as shown in FIG. 14 . The work functionadjustment layer 73 is made of a conductive material such as a singlelayer of TiN, TaN, TaAlC, TiC, TaC, Co, Al, TiAl, HfTi, TiSi, TaSi orTiAlC, or a multilayer of two or more of these materials. For then-channel Fin FET, one or more of TaN, TaAlC, TiN, TiC, Co, TiAl, HfTi,TiSi and TaSi is used as the work function adjustment layer, and for thep-channel Fin FET, one or more of TiAlC, Al, TiAl, TaN, TaAlC, TiN, TiCand Co is used as the work function adjustment layer.

The metal gate electrode layer 74 is formed over the one or more workfunction adjustment layers 73. The metal gate electrode 74 includes oneor more layers of any suitable metal material, such as aluminum, copper,titanium, tantalum, cobalt, molybdenum, tantalum nitride, nickelsilicide, cobalt silicide, TiN, WN, TiAl, TiAlN, TaCN, TaC, TaSiN, metalalloys, other suitable materials, and/or combinations thereof.

After depositing appropriate materials for the metal gate structure,planarization operations, such as CMP, are performed. Further, in someembodiments, the metal gate structure is recessed to form a cap space,and the cap space is filled with an insulating material 78, as shown inFIG. 14 .

After the metal gate structure is formed, the first ILD layer 67 anddielectric cover layer 65 are removed. In some embodiments, the firstILD layer 67 and dielectric cover layer 65 are fully removed and inother embodiments, the first ILD layer 67 and dielectric cover layer 65are partially removed from an area around the source/drain structures.

After the first ILD layer 67 is at least partially removed, metal alloylayers 80 and 81 are respectively formed over the source/drain epitaxiallayers 60 and 62, as shown in FIG. 15 . The respective step is shown asstep S17 in the process flow in FIG. 1 .

The metal alloy layers 80 are 81 alloys made of one or more Group IVelement and one or more transition metal elements. When the source/drainepitaxial layers 60 and 62 are formed by silicon, the metal alloy layers80 and 81 are silicide layers. When the source/drain epitaxial layers 60and 62 are formed by germanium, the metal alloy layers 80 and 81 aregermanide layers. When the source/drain epitaxial layer 60 and 62 areformed by SiGe, the metal alloy layer 80 and 81 are silicide-germanidelayers.

The transition metal includes one or more of Ti, Ta, Ru, Pt, Ni and Co.The alloy layer 80 or 81 is one or more of TiSi, TaSi, RuSi, PtSi, NiSi,CoSi, TiSiGe, TaSiGe, NiSiGe and CoSiGe.

After the first ILD layer 67 is removed, transition metal is depositedby, for example, CVD, ALD or PVD, on the source/drain epitaxial layers60 and 62. During the deposition, the deposited transition metal reactswith Si and/or Ge in the source/drain epitaxial layers 60 and 62,thereby forming the alloy layers 80 and 81 in some embodiments. In someembodiments, silicide (alloy) layer can be formed by PECVD, CVD, PEALD,or ALD metal deposit in a temperature range of about 250 to 700° C., andthen an in-situ dry etching using Cl-based or F-based gas or ex-situ wetselective etching is applied to remove the remaining metal on spacer andthe isolation insulating layer. In other embodiments, the silicide(alloy) layer can be formed by PECVD, CVD, PEALD, or ALD metal depositin a temperature range of about 350 to 650° C. In some embodiments, anitridation treatment is later performed to passivate silicide surfacefor the subsequent silicide formation anneal. In other embodiments, aselective silicide deposition process through surface blocking byself-assembly molecular (SAMs), or inherent selective formation fromproper metal and silicon precursors is performed. Other suitablesilicide formation processes may be utilized.

In the present embodiments, before the alloy layers 80 are formed, thesource/drain epitaxial layer 60 on one of the fin structures 52 isseparated from the source/drain epitaxial layer 60 on the adjacent finstructure 52, and the source/drain epitaxial layer 62 on one of the finstructures 54 is separated from the source/drain epitaxial layer 62 onthe adjacent fin structure 54. The alloy layers 80 and 81 are formedsuch that the resulting alloy layers 80 on the separated source/drainepitaxial layers 60 are separated, and the resulting alloy layers 81 onthe separated source/drain epitaxial layers 62 are separated as well.

In some embodiments, after a transition metal layer is formed on thesource/drain epitaxial layers 60 and 62, an annealing operation isperformed to form the alloy layers 80 and 81. The annealing operation isperformed at a temperature of about 250° C. to about 850° C.

After the alloy layers 80 and 81 are formed, a contact-etch stop layer(CESL) 150 is formed to cover the alloy layer 80, 81, and a second ILDlayer 160 is formed on the CESL 150, as shown in FIG. 16 . The CESL 150is made of a silicon nitride based material, such as SiN and SiON, andhas a thickness in a range from about 2 nm to about 20 nm in someembodiments. The second ILD 160 is made of a different material than theCESL 150 and is made of, for example, one or more layers of siliconoxide, SiCN, SiOCN or a low-k material.

Subsequently, a patterning operation is performed to form contactopenings over the alloy layers 80 and 81 of the source/drain structures,and the openings are filled with a conductive material, thereby formingcontact plugs 200 and 201, as shown in FIG. 16 . The respective step isshown as step S18 in the process flow in FIG. 1 . The contact plugs 200and 201 are formed using a selective bottom-up deposition technique tofill a gap between the neighboring alloy layers, which will be describedfurther below.

FIGS. 17A-20B are exemplary cross-sectional views at various stages offorming contact plugs according to some embodiments of the presentdisclosure. In FIGS. 17A-20B, the “A” figures (e.g., FIG. 17A)illustrate a cross-sectional view along X direction corresponding to theline A-A illustrated in FIG. 16 , and the “B” figures (e.g., FIG. 17B)illustrate a cross-sectional view along the Y direction corresponding tothe line B-B illustrated in FIG. 16 . In FIGS. 17A-20B, an n-channel FET(NFET) and a p-channel FET (PFET), each of which includes two finstructures 52, 54 are illustrated as adjacent to each other. However,the configuration is not limited to this. The number of the finstructures may be three or more and one or more additional structuresmay be disposed between the NFET and the PFET.

In FIGS. 17A and 17B, in the n-channel FET (NFET), a first n-typeepitaxial layer 132 is formed on the fin structure 52, and a secondn-type epitaxial layer 134 is formed on the first n-type epitaxial layer132. The first and second n-type epitaxial layers are crystallinesemiconductor layers, such as Si, SiC, SiCP, SiP, Ge and SiGe, havingdifferent lattice constants from each other and from the fin structure.When SiC, SiP and/or SiCP are used, the C or P concentration of thesecond n-type epitaxial layer 134 is higher than that of the firstn-type epitaxial layer 132. In some embodiments, a Group III-Vsemiconductor layer is used for at least one of the first and secondn-type epitaxial layers. The combination of the first and second n-typeepitaxial layers 132 and 134 correspond to the source/drain epitaxiallayer 60 shown in FIG. 16 . In other embodiments, only one of the firstand second n-type epitaxial layers 132 and 134 is formed, and in certainembodiments, three or more n-type epitaxial layers are formed.

Further, the alloy layers 222, for example, silicide layers, whichcorrespond to the alloy layers 80 shown in FIG. 16 , are formed on thesecond n-type epitaxial layers 134, respectively. The alloy layers 222are formed by a reaction between the material of the second n-typeepitaxial layer 134 and a transition metal layer formed thereon. Asshown in FIG. 17A, the second n-type epitaxial layer 134 of one of theadjacent two fin structures 52 does not touch the second n-typeepitaxial layer 134 of the other one of the adjacent two fin structures52. The alloy layers 222 formed on the respective second n-typeepitaxial layers 134 are not merged and thus separated.

In some embodiments, a surface layer of the alloy layer 222 can beconverted to a nitridated alloy cap 215 by a nitridation operation byusing NH₃ or N₂+H₂ plasma in some embodiments. The nitridated alloy cap215 covers the surface of the alloy layer 222 to protect the alloy layer222 from damages caused by the subsequent processes, such as metaldiffusion from the subsequently formed contact plug 200. The nitridatedalloy caps 215 on the neighboring fin structures 52 each include anup-slant sidewall SW1 and a vertical sidewall SW2 extending downwardlyfrom a bottom of the up-slant sidewall SW1 along a directionnon-parallel to the up-slant sidewall SW1. This is due to the fact thatthe nitridated alloy cap 215 wraps around facets of the epitaxial layer134 extending along different crystal planes (i.e., having differentcrystal orientations).

A transition metal layer 225 remains at the bottom of the void 224formed by the alloy layer 222 and the isolation insulating layer 58. Thevolume of the transition metal layer 225 is sufficiently small so as notto affect electrical properties of the NFET. Further, spacer residues110, which are a remaining part of the sidewall spacers 24 that is notetched in the operation of FIG. 11 , exist.

The p-channel FET (PFET) shown in FIG. 17A has the same or similarstructure as the n-channel FET (NFET), and explanations for the commonconfigurations may be omitted. A first p-type epitaxial layer 136 isformed on the fin structure 54, and a second p-type epitaxial layer 138is formed on the first p-type epitaxial layer 136. The first and secondp-type epitaxial layers are crystalline semiconductor layers, such asSi, SiC, SiCP, SiP, Ge and SiGe, having different lattice constants fromeach other and from the fin structure. When SiGe is used, the Geconcentration of the second p-type epitaxial layer 138 is higher thanthat of the first p-type epitaxial layer 136. In some embodiments, aGroup III-V semiconductor layer is used for at least one of the firstand second p-type epitaxial layers. The combination of the first andsecond p-type epitaxial layers 136 and 138 correspond to thesource/drain epitaxial layer 62 shown in FIG. 16 . In other embodiments,only one of the first and second p-type epitaxial layers 136 and 138 isformed, and in certain embodiments, three or more p-type epitaxiallayers are formed.

Further, the alloy layers 220, for example, silicide layers, whichcorrespond to the alloy layers 81 shown in FIG. 16 , are formed on therespective second p-type epitaxial layers 138. The alloy layer 220 isformed by a reaction between the material of the second p-type epitaxiallayer 138 and a transition metal layer formed thereon. As shown in FIG.17A, the second p-type epitaxial layer 138 of one of the adjacent twofin structure 54 does not touch the second p-type epitaxial layer 138 ofthe other one of the adjacent two fin structure 54. The alloy layers 220formed on the respective second p-type epitaxial layers 138 are notmerged and thus separated.

In some embodiments, a surface layer of the alloy layer 220 can beconverted to a nitridated alloy cap 216 by the nitridation operation asdiscussed previously with respect to formation of the nitridated alloycap 215. The nitridated alloy cap 216 covers the surface of the alloylayer 222 to protect the alloy layer 222 from damages caused by thesubsequent processes, such as metal diffusion from the contact plug 201.The nitridated alloy caps 216 on the neighboring fin structures 54 eachinclude an up-slant sidewall SW3 and a vertical sidewall SW4 extendingdownwardly from a bottom of the up-slant sidewall SW3 along a directionnon-parallel to the up-slant sidewall SW3. This is due to the fact thatthe nitridated alloy cap 216 wraps around facets of the epitaxial layer138 extending along different crystal planes.

A transition metal layer 225 remains at the bottom of the void 226formed by the alloy layer 220 and the isolation insulating layer 58. Thevolume of the transition metal layer 225 is sufficiently small so as notto affect electrical properties of the PFET. Further, spacer residues110, which are a remaining part of the sidewall spacers 24 that is notetched in the operation of FIG. 11 , exist.

The contact etch stop layer (CESL) 150 is formed as a blanket layer tocover an entirety of the NFET and PFET, and the second ILD layer 160 isform to cover an entirety of the CESL 150. Contact openings O1 and O2are then etched in the second ILD layer 160 and through the CESL 150 toexpose the nitridated alloy caps 215 and 216, respectively. Theresulting structure is illustrated in FIGS. 18A and 18B. The contactopening O1 includes a tapered opening O11 and a gap O12 extendingdownwardly from a bottom of the tapered opening O11. The gap O12 isdefined by vertical sidewalls SW2 of the nitridated alloy cap 215, andthe tapered opening O11 is defined by up-slant sidewalls SW1 of thenitridated alloy cap 215 and up-slant sidewalls SW5 of the second ILDlayer 160 having different slopes than up-slant sidewalls SW1 of thenitridated alloy cap 215. Similarly, the contact opening O2 includes atapered opening O21 and a gap O22 extending downwardly from a bottom ofthe tapered opening O21. The gap O22 is defined by vertical sidewallsSW4 of the nitridated alloy cap 216, and the tapered opening O21 isdefined by up-slant sidewalls SW3 of the nitridated alloy cap 216 andup-slant sidewalls SW6 of the second ILD layer 160 having differentslopes than the up-slant sidewalls SW3. The width of the gap O21 or O22is in a range from about 3 nm to about 15 nm in some embodiments. Thewidth of the gap O12 or O22 is controlled to facilitate the followingbottom-up deposition using the capillary condensation mechanism, whichwill be discussed in detail below. If the width of the gap O21 or O22 isgreater than about 15 nm or less than about 3 nm, it might adverselyaffect the following bottom-up deposition process.

The contact plugs 200 and 201 are then formed in the respective contactopenings O1 and O2, as illustrated in FIGS. 19A and 19B. The contactplugs 200 and 201 are deposited using a bottom-up approach, so that thenarrow gaps O12 and O22 (as shown in FIG. 18A) can be substantiallyfilled with the contact plugs 200 and 201, respectively. In certainembodiments, the bottom-up deposition is a CVD deposition processinvolving a capillary condensation mechanism and thus is equivalentlyreferred to as a capillary condensation CVD process hereinafter.

The capillary condensation CVD process selectively deposits metal in thenarrowest confined spaces on the substrate, such as the gaps O12 and O22of the contact openings O1 and O2. The precursor of the capillarycondensation CVD process is initially provided in a vapor phase at apartial pressure (e.g., from 0.01 mT to about 0.1 mT) below itssaturation pressure. If the partial pressure of the precursor is greaterthan about 0.1 mT or less than about 0.01 mT, the capillary condensationmechanism might be unsatisfactory. Due to capillary condensationmechanism, the precursor vapor condenses as a liquid in the narrowestgaps (i.e., the gaps O12 and O22) even if the partial pressure of theprecursor vapor is below its saturation pressure. This is due to anincreased number of van der Waals interactions between vapor phasemolecules inside the confined space of capillaries (i.e., the gaps O12and O22). This propensity is employed to selectively fill the narrowgaps O12 and O22 between the non-merged alloy layers with the precursorliquid. With the precursor liquid selectively confined to narrowestgaps, the precursor liquid is converted into a solid-phase depositedmetal. This conversion may be achieved physically for example bysolidification, or chemically by reacting the precursor liquid withanother material to produce the solid material, or by decomposing theprecursor.

Metal deposition using the capillary condensation CVD begins from thenarrowest confined spaces, which in turn results in a “bottom-up”filling phenomenon instead of a “conformal” deposition phenomenon, thussubstantially filling the gaps O12 and O22 with the contact plugs 200and 201. In this way, the narrow gap between vertical sidewalls of thenitridated alloy caps can be filled by metal, not by silicide, germanideor metal nitride, which in turn will achieve reduced contact resistancebecause metal has lower resistance than silicide, germanide or metalnitride. In some embodiments, the capillary condensation CVD process isperformed until that the contact openings O1 and O2 are overfilled withmetal, as illustrated in FIGS. 19A and 19B.

In some embodiments, the precursor of the capillary condensation CVD maybe, for example, metal carbonyl, metal halide, the like or combinationsthereof, and the carrier gas may include, for example, argon gas,hydrogen gas, the like, or combinations thereof. In some embodiments, anadditional inhibition gas is employed during the capillary condensationCVD process. The inhibition gas can improve “bottom-up” fillingphenomenon because it inhibits metal nucleation on dielectric (e.g., onsidewalls of tapered openings O11 and O21 in the ILD layer 160). In someembodiments where the precursor includes metal carbonyl, the inhibitiongas includes, for example, carbon monoxide (CO). In some embodimentswhere the precursor includes metal halide, the inhibition gas includes,for example, hydrogen halide.

In some embodiments, noble metals (e.g., ruthenium) are promisingcandidates for the contact plugs 200 and 201 compared to non-noblemetals such as copper. This is at least due to that the noble metalsexhibit lower metal diffusion than non-noble metals (e.g., copper). As aresult, a diffusion barrier layer deposited prior to formation of thecontact plugs 200 and 201 can be omitted if the contact plugs 200 and201 include the noble metal (e.g., ruthenium). Omission of the diffusionbarrier will contributes to an improved process window for depositingthe contact plugs 200 and 201 into the narrow gaps O12 and O22.

At an initial stage of the deposition, the noble metal (e.g., ruthenium)may react with silicon and/or germanium in the exposed regions (e.g.,silicon and/or germanium in the silicon-containing and/orgermanium-containing nitridated alloy caps 215 and 216 and in thesidewalls of contact openings O1 and O2 in the silicon-containing ILDlayer 160), thereby forming interfacial layers 241 and 242 that containthe noble metal element and the Group IV element (e.g., silicon and/orgermanium). At a following stage of the deposition, the noble metal isnot reacted with silicon and/or germanium, which in turn results insilicon-free and/or germanium-free noble metal structures 251 and 252formed on the respective interfacial layers 241 and 242. In this way,the interfacial layer 241 improves adhesion of the silicon-free and/orgermanium-free noble metal structure 251 to the nitridated alloy cap 215and ILD layer 160. Similarly, the interfacial layer 242 improvesadhesion of the silicon-free and/or germanium-free ruthenium structure252 to the nitridated alloy cap 215 and ILD layer 160.

In some embodiments, a pre-clean process is performed prior to thecapillary condensation CVD process. The pre-clean includes ionbombardment using hydrogen (H₂), argon, helium, nitrogen (N₂), the like,or combinations thereof. As a result of the bombardment, bonds of thedielectric materials (such as silicon oxide and/or nitride) exposed bythe contact openings O1 and O2 may be broken or loosen, which willfacilitate formation of the interfacial layers 241 and 242 by reactingthe noble metal with silicon resulting from the broken bonds in thedielectric materials.

In some embodiments where the noble metal is ruthenium, the precursormay be, for example, ruthenium carbonyl, ruthenium tricarbonyl,ruthenium halide, the like or combinations thereof, and the carrier gasmay include, for example, argon gas, hydrogen gas, the like orcombinations thereof.

In some embodiments where the precursor is ruthenium tricarbonyl,formation of contact plugs 200 and 201 may be a cyclic process includingone or more repetitions of a deposition cycle and a cleaning cycle. Forexample, it may perform a deposition cycle followed by a cleaning cycle,and repeats the deposition and cleaning cycles. In some embodiments, thedeposition cycle involves the capillary condensation CVD process asdiscussed previously. In the deposition cycle, ruthenium tricarbonylprecursor is initially provided in a vapor phase at a partial pressure(e.g., from 0.01 mT to about 0.1 mT) below its saturation pressure. Ifthe partial pressure of the ruthenium trycarbonyl is greater than about0.1 mT or less than about 0.01 mT, the capillary condensation mechanismmight be unsatisfactory. The cleaning cycle involves treating thecontact openings O1 and O2 with ion bombardment performed using a gasincluding hydrogen (H₂), argon, helium, nitrogen (N₂), the like, orcombinations thereof. The ion bombardment is advantageous for formingthe interfacial layers 241 and 242 on the sidewalls of the contactopenings O1 and O2 as discussed previously. In some embodiments, thedeposition cycle and/or the cleaning cycle are performed at atemperature in a range from about 160° C. to about 250° C. If thetemperature of the deposition cycle and/or the cleaning cycle is out ofthe range from about 160° C. to about 250° C., the bottom-up fillingphenomenon might be degraded, thus leading to voids in the resultingconductive plugs 200 and 201.

In some embodiments where the precursor is ruthenium carbonyl, formationof the contact plugs 200 and 201 may be a single-step CVD processinvolving the capillary condensation mechanism as discussed above. TheCVD process is performed at a temperature in a range from about 130° C.to about 250° C. If the temperature of the thermal CVD process is out ofthe range from about 130° C. to about 250° C., the bottom-up fillingphenomenon might be degraded, thus leading to voids in the resultingconductive plugs 200 and 201. Ruthenium carbonyl is initially providedin a vapor phase at a partial pressure (e.g., from 0.01 mT to about 0.1mT) below its saturation pressure. If the partial pressure of theruthenium carbonyl is greater than about 0.1 mT or less than about 0.01mT, the capillary condensation mechanism might be unsatisfactory.

Ruthenium-containing precursors as discussed above are not only suitablefor implementing the capillary condensation mechanism, but also haveinherent deposition selectivity between metal and oxide if thedeposition process is performed at a temperature lower than about 250°C., as discussed below.

FIG. 27 illustrates experimental results of depositing ruthenium onvarious materials using a CVD process performed at a temperature rangingfrom about 195° C. to about 205° C. with ruthenium tricarbonyl as aprecursor. FIG. 27 is a chart showing the thickness of the rutheniumlayer as a function of time, wherein the thickness of the rutheniumlayer is measured using X-ray fluorescence (XRF) and expressed as XRFintensity. In Condition #1, the CVD process is carried out to depositruthenium on platinum. In Condition #2, the CVD process is carried outto deposit ruthenium on silicon oxide. When the CVD process usingruthenium tricarbonyl as a precursor is performed at a temperatureranging from about 195° C. to about 205° C., the thickness of theruthenium layer deposited on platinum is greater than that deposited onsilicon oxide (comparing Condition #1 with Condition #2), which meansthat ruthenium tricarbonyl has deposition selectivity between metal andoxide if the deposition temperature is at a temperature in a range fromabout 195° C. to about 205° C.

FIG. 28 illustrates experimental results of depositing ruthenium onvarious materials using a CVD process performed at a temperature rangingfrom about 250° C. to about 260° C. with ruthenium tricarbonyl as aprecursor. FIG. 28 is a chart showing the thickness of the rutheniumlayer as a function of time, wherein the thickness of the rutheniumlayer is measured using scanning electron microscope (SEM). In Condition#3, the CVD process is carried out to deposit ruthenium on siliconoxide. In Condition #4, the CVD process is carried out to depositruthenium on ruthenium. In Condition #5, the CVD process is performed todeposit ruthenium on platinum. When the CVD process using rutheniumtricarbonyl as a precursor is performed at a temperature in a range fromabout 250° C. to about 260° C., the thickness of the ruthenium layerdeposited on silicon oxide is substantially the same as or slightlygreater than that deposited on platinum and ruthenium (comparingCondition #3 with Conditions #4 and #5,), which means rutheniumtricarbonyl has unsatisfactory selectivity between metal and oxide ifthe deposition temperature is equal to or greater than about 250° C.Because of the experimental results from FIGS. 27 and 28 , thedeposition temperature of formation of the contact plugs 200 and 201 maybe not higher than about 250° C., so that the contact plugs 200 and 201can be selectively deposited on the respective alloy caps 215 and 216,which in turn will result in improved bottom-up deposition.

Returning to FIGS. 19A and 19B, after depositing the contact plugs 200and 201, an optional anneal process is performed to reduce seams 261 and262 in the noble metal structures 251 and 252, and an optional CMPprocess is carried out to remove excess materials of the noble metalstructures 251, 252, and interfacial layers 241, 242 outside the contactopenings O1 and O2. The resulting structure is shown in FIGS. 20A and20B. In some embodiments, the anneal process is performed at atemperature of about 350° C. to about 500° C. for a duration of about 2minutes to 45 minutes. If the temperature is greater than 500° C. and/orduration is greater than 45 minutes, it might lead to adverse impact onwork functions of the metal gates (e.g., work functions of the workfunction adjustment layers 73 and the metal gate electrode layers 74).If the temperature is less than 350° C. and/or duration is greater than2 minutes, the seams 261 and 262 might remain in the noble metalstructures 251 and 252 after the anneal process.

As illustrated in FIG. 20A, the contact plug 200 is in contact with theup-slant sidewalls SW1 and the vertical sidewalls SW2 of the nitridatedalloy caps 215. In some embodiments, the contact plug 200 includes atapered portion 2001 tapering toward the substrate 50 and an extendingportion 2002 extending downwardly from a bottom of the tapered portion2001. The tapered portion 2001 and the extending portion 2002 arerespectively in contact with the up-slant sidewall SW1 and the verticalsidewall SW2 of the nitridated alloy cap 215. The tapered opening O11and the gap O12 are filled with the tapered portion 2001 and theextending portion 2002, respectively. Therefore, the tapered portion2001 and the extending portion 2002 have contours the same as thetapered opening O11 and the gap O12, respectively. Specifically, thetapered portion 2001 has a width variation greater than a widthvariation of the extending portion 2002. For example, the width of thetapered portion 2001 decreases as a distance from the substrate 50decreases, and the width of the extending portion 2002 remainssubstantially constant as the distance from the substrate 50 decreases.

Similarly, the contact plug 201 is in contact with the up-slantsidewalls SW3 and the vertical sidewalls SW4 of the nitridated alloycaps 216. In some embodiments, the contact plug 201 includes a taperedportion 2011 tapering toward the substrate 50 and an extending portion2012 extending downwardly from a bottom of the tapered portion 2011. Thetapered portion 2011 and the extending portion 2012 are respectively incontact with the up-slant sidewall SW3 and the vertical sidewall SW4 ofthe nitridated alloy cap 216. The tapered opening O21 and the gap O22are filled with the tapered portion 2011 and the extending portion 2012,respectively. Therefore, the tapered portion 2011 and the extendingportion 2012 have contours the same as the tapered opening O21 and thegap O22, respectively.

FIGS. 21A-23B are exemplary cross-sectional views at various stages offorming contact plugs according to some embodiments of the presentdisclosure. In FIGS. 21A-23B, the “A” figures (e.g., FIG. 21A)illustrate a cross-sectional view along X direction corresponding to theline A-A illustrated in FIG. 16 , and the “B” figures (e.g., FIG. 21B)illustrate a cross-sectional view along the Y direction corresponding tothe line B-B illustrated in FIG. 16 . FIGS. 21A-23B shows substantiallythe same structures as FIGS. 17A-20B, except for the contact plugs. Asillustrated in FIGS. 21A and 21B, the noble metal structures 251 and 252are respectively deposited in lower parts of the contact openings O1 andO2, and upper parts of the contact openings O1 and O2 are free from thenoble metal structures 251 and 252 and remain unfilled.

Afterwards, seed layers 271 and 272 are respectively formed in the upperparts of the contact openings O1 and O2 and on the noble metalstructures 251 and 252, followed by depositing conductive structures,such as metal structures 281 and 282, on the respective seed layers 271and 272 to form contact plugs 200 a and 201 a, as illustrated in FIGS.22A and 22B. In some embodiments, the seed layers 271 and 272 aredifferent portions of a blanket seed layer conformally deposited on thesubstrate 50, and the metal structures 281 and 282 are different portionof a metal layer deposited on the blanket seed layer. Thereafter, anoptional anneal process can be performed to reduce the seams 261 and 262in the noble metal structures 251 and 252, and a CMP is carried out toremove excess materials of the metal structures 281, 282, and seedlayers 271, 272 outside the contact openings O1, O2. The resultingstructure is shown in FIGS. 23A and 23B.

As illustrated in FIG. 23A, the seed layers 271 and 272 are conformallydeposited in the upper parts of the tapered openings O11 and O21 andthus have a substantially U-shaped profile. Remaining upper parts of thetapered openings O11 and O21 are filled with the respective metalstructures 281 and 282. Therefore, the metal structures 281 and 282 havetapered profile corresponding to the tapered openings O11 and O21, andcan be equivalently referred to as tapered portions of the contact plugs200 a and 201 a.

Materials of the seed layers 271, 272 and the metal structures 281, 282are selected such that the metal structures 281 and 282 are deposited ata faster deposition rate than that of the capillary condensation CVD fordepositing the noble metal structures 251 and 252. In this way, thecontact plugs 200 a and 201 a can be formed in a shortened duration. Insome embodiments, the seed layers 271 and 272 are made of TiN, TaN, thelike, or combinations thereof, and the metal structures 281 and 282 madeof, for example, Co, W, Ti, Ta, Cu, Rh, Ir, Al and/or Ni, different fromthe noble metal structures 251 and 252. These materials of seed layers271 and 272 not only result in improved deposition rate of depositingthe metal structures 281 and 282, but also act as diffusion barriers tosuppress out-diffusion of metals of the metal structures 281 and 282into the second ILD layer 160. As a result, the seed layers 217 and 272can be equivalently referred to as diffusion barrier layers in someembodiments.

As shown in FIG. 23A, the diffusion barrier layer 271 extends in thecontact opening O1 and terminates prior to reaching the alloy cap 215,and diffusion barrier layer 272 extends in the contact opening O2 andterminates prior to reaching the alloy cap 216. Stated differently, thediffusion barrier layer 271 over the noble metal structure 251 isseparated from the alloy cap 215, and the diffusion barrier layer 272over the noble metal structure 252 is separated from the alloy cap 216.As a result, the diffusion barrier layer 271 does not extend into thenarrow gap defined by the vertical sidewalls SW2 of the nitridated alloycaps 215, and the diffusion barrier layer 272 does not extend into thenarrow gap defined by the vertical sidewalls SW4 of the nitridated alloycaps 216.

FIGS. 24A and 24B are exemplary cross-sectional views of a semiconductordevice according to some embodiments of the present disclosure. FIG. 24Aillustrates a cross-sectional view along X direction corresponding tothe line A-A illustrated in FIG. 16 , and FIG. 24B illustrates across-sectional view along the Y direction corresponding to the line B-Billustrated in FIG. 16 . FIGS. 24A-24B show substantially the samestructures as FIGS. 20A-20B, except there is a contact plug 203 landingon a single alloy cap 215. In some embodiments, the contact plug 203includes an interfacial layer 243 and a noble metal structure 253 on theinterfacial layer 243, which are substantially the same as theinterfacial layer 241 and the noble metal structure 251, respectively,as discussed previously. Moreover, a void 263 is formed between thecontact plug 203 and the second ILD layer 160. This is due to that thecapillary condensation CVD process exhibits lower nucleation on oxide ofthe second ILD layer 160 than on metal of the alloy cap 215.

FIGS. 25 and 26 are exemplary cross-sectional views at various stages offorming contact plugs according to some embodiments of the presentdisclosure. FIG. 25 shows substantially the same structure as FIG. 15 ,except for the interfacial layers 244 and 245 formed on the respectivealloy caps 215 and 216, and noble metal structures 254 and 255 formed onthe respective interfacial layers 244 and 245.

As illustrated in FIG. 25 , the interfacial layers 244 respectively wraparound the separated alloy caps 215 on the neighboring fin structures52, and a noble metal structure 254 wrap around the interfacial layers244. The interfacial layers 244 and noble metal structure 254 aresubstantially the same as the interfacial layer 241 and the noble metalstructure 251 formed using the capillary condensation CVD, as discussedpreviously. Similarly, interfacial layers 245 respectively wrap aroundthe separated alloy caps 216 on the neighboring fin structures 54, and anoble metal structure 255 wraps around the interfacial layers 245. Theinterfacial layers 245 and noble metal structure 255 are substantiallythe same as the interfacial layer 242 and the noble metal structure 252formed using the capillary condensation CVD, as discussed previously.

Afterwards, the second ILD layer 160 is formed over the noble metalstructures 254 and 255, as illustrated in FIG. 26 . In some embodiments,the noble metal structure 254 and the interfacial layers 244 in thenoble metal structure 254 can in combination serve as a contact plug 200b for the neighboring semiconductor fins 52, and in other embodiments,an additional metal structure is formed in the ILD layer 160 and on thenoble metal structure 254 to complete formation of the contact plug 200b for the semiconductor fins 52. Similarly, in some embodiments, thenoble metal structure 255 and the interfacial layers 245 in the noblemetal structure 255 can in combination serve as a contact plug 201 b forthe neighboring semiconductor fins 54, and in other embodiments, anadditional metal structure is formed in the ILD layer 160 and on thenoble metal structure 255 to complete formation of the contact plug 201b.

Based on the above discussions, it can be seen that the presentdisclosure offers advantages. It is understood, however, that otherembodiments may offer additional advantages, and not all advantages arenecessarily disclosed herein, and that no particular advantages arerequired for all embodiments. One advantage is that the reducedsource/drain contact resistance and hence improved device performance isachieved by forming a metal in the narrow gap between alloy layers(e.g., silicide layers) on neighboring semiconductor fins. Anotheradvantage is that a bottom-up deposition can be achieved to fill thenarrow gap between the alloy layers by using a capillary condensationCVD process. Another advantage is that enhanced bottom-up deposition andhence reduced volume of seams can be achieved by using an inhibition gasto inhibit metal nucleation on dielectric materials during the capillarycondensation CVD. Another advantage is that the out-diffusion of themetal of the contact plug can be suppressed by the nitridated silicidecap. Another advantage is that reduced metal diffusion and henceomission of diffusion barrier metal can be achieved by using noble metalto form the contact plug.

In some embodiments, a semiconductor device comprises a firstsemiconductor fin, a first epitaxial layer, a first alloy cap and acontact plug. The first semiconductor fin is on a substrate. The firstepitaxial layer is on the first semiconductor fin. The first alloy capis over the first epitaxial layer. The first alloy cap is made of one ormore Group IV elements and one or more metal elements, and the firstalloy cap comprises a first sidewall and a second sidewall extendingdownwardly from a bottom of the first sidewall along a directionnon-parallel to the first sidewall. The contact plug is in contact withthe first and second sidewalls of the first alloy cap.

In some embodiments, a semiconductor device comprises a firstsemiconductor fin, a first epitaxial layer, a first alloy cap and acontact plug. The first semiconductor fin is on a substrate. The firstepitaxial layer is on the first semiconductor fin. The first alloy capis over the first epitaxial layer. The first alloy cap is made of one ormore Group IV elements and one or more metal elements. The contact plugcomprises an interfacial layer in contact with the first alloy cap, anda noble metal structure in contact with the interfacial layer, whereinthe interfacial layer comprises the one or more Group IV elements and anoble metal element of the noble metal structure.

In some embodiments, a method for forming a semiconductor devicecomprises forming an epitaxial layer on a semiconductor fin, forming analloy layer on the epitaxial layer by reacting the epitaxial layer witha metal, and forming a first metal structure on the alloy layer using adeposition process. The deposition process comprises providing aprecursor in a vapor phase and keeping a partial pressure of theprecursor below a saturation pressure of the precursor such that theprecursor condenses as a liquid on the alloy layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A device, comprising: a first semiconductor finand a second semiconductor fin extending from a substrate; a firstepitaxial layer wrapping around the first semiconductor fin; a secondepitaxial layer wrapping around the second semiconductor fin; and acontact plug over the first epitaxial layer and the second epitaxiallayer, the contact plug comprising a first interfacial layer over thefirst epitaxial layer and a second interfacial layer over the secondepitaxial layer, the first and second interfacial layers comprising anoble metal element and a Group IV element.
 2. The device of claim 1,wherein the noble metal element is ruthenium.
 3. The device of claim 1,wherein the contact plug comprises a lower portion and an upper portionover the lower portion, wherein the upper portion has a width variationgreater than a width variation of the lower portion.
 4. The device ofclaim 3, wherein sidewalls of the lower portion are more vertical thansidewalls of the upper portion.
 5. The device of claim 1, wherein amajority of the contact plug is the noble metal element.
 6. The deviceof claim 1, further comprising: a first nitridated metal-containinglayer between the first epitaxial layer and the first interfacial layer.7. The device of claim 6, wherein the first interfacial layer is incontact with the first nitridated metal-containing layer.
 8. The deviceof claim 6, further comprising: a second nitridated metal-containinglayer between the second epitaxial layer and the second interfaciallayer.
 9. The device of claim 8, wherein the second interfacial layer isin contact with the second nitridated metal-containing layer.
 10. Thedevice of claim 1, wherein the first interfacial layer and the secondinterfacial layer are formed of a single continuous layer.
 11. A device,comprising: a semiconductor fin above a substrate; a gate structure overthe semiconductor fin; a gate spacer on a sidewall of the gatestructure; a source/drain epitaxial region spaced apart from the gatestructure by the gate spacer; and a source/drain contact over thesource/drain epitaxial region, the source/drain contact comprising afirst metal region over the source/drain epitaxial region, a seed layerover the first metal region, and a second metal region spaced apart fromthe first metal region by the seed layer, wherein a top position of thefirst metal region is lower than a top position of the second metalregion.
 12. The device of claim 11, wherein the second metal regioncomprises a different material than the first metal region.
 13. Thedevice of claim 11, wherein the first metal region comprises a noblemetal, and the second metal region comprises a non-noble metal.
 14. Thedevice of claim 11, wherein the first metal region is a barrier-lessmetal region formed of a homogeneous material.
 15. The device of claim14, wherein the homogeneous material comprises ruthenium.
 16. The deviceof claim 11, further comprising: an interfacial layer between thesource/drain epitaxial region and the first metal region, wherein theinterfacial layer comprises a Group IV element and a metal element, andthe metal element is different from a metal element within the secondmetal region.
 17. A device, comprising: a semiconductor fin above asubstrate; a source/drain epitaxial region over the semiconductor fin;an interlayer dielectric (ILD) layer over the source/drain epitaxialregion; a source/drain contact over the source/drain epitaxial region,the source/drain contact extending in the ILD layer; and a void regionbetween the ILD layer and a bottom corner of the source/drain contact.18. The device of claim 17, further comprising: an interfacial layerbetween the source/drain epitaxial region and the source/drain contact,the interfacial layer comprising a metal element and a Group IV element.19. The device of claim 18, wherein the void region is free of theinterfacial layer.
 20. The device of claim 17, wherein the source/draincontact is a barrier-less metal region formed of a homogeneous material.